Franz Weidenreich's reconstructions of classic Indonesian and Chinese Homo erectus skulls (redrawn by Kathryn Cruz-Uribe partly after originals by Janis Cirulis in W. W. Howells 1967, Mankind in the Making. New York: Doubleday, pp. 156, 169).
purchases and resells fossils and ancient artifacts. The owner recognized the skullcap for what it was, and he made it available to scientists at the American Museum of Natural History. It was subsequently returned to Indonesia for curation. Still, in the most extreme way imaginable, the New York City discovery illustrates a problem that besets all the Javan fossils, beginning with Dubois's original find—their stratigraphic context was not carefully documented in the field. Sometimes, even the precise find spots are uncertain, because the discoverers were farmers who sold the fossils to scientists.
Java is a land of volcanoes, and in theory, it offers the same potential to date fossils as eastern Africa, since the fossil-bearing deposits often contain volcanic rock fragments or ash layers that are amenable to potassium/argon dating. In some places, the deposits also contain tektites—glassy rocks of meteoritic origin that were molten before they hit Earth and that can be dated in the same way as lava or ash. The dates on various Javan materials range from 2 million to 470,000 years ago, but their meaning is difficult to assess, since the stratigraphic relationship of the materials to the fossils and to each other is largely unknown.
Garniss Curtis, director of the Berkeley Geochronology Center, and his colleague, Carl Swisher, now at Rutgers University, have produced the most credible and most widely publicized dates. Curtis was instrumental in the revolutionary potassium/argon dating of Olduvai Gorge and other east African sites in the 1960s, and he made his first attempt in Java in 1974. He collected a sample of volcanic rock from the vicinity of the Mojokerto site that produced the child's skull in 1936, and he obtained an age of 1.9 million years ago. However, few authorities took this date seriously, mainly because the stratigraphic relationship between the dated sample and the skull was unclear.
In 1992-93, Curtis returned to Java with Swisher for another try. They collected fresh volcanic samples from Mojokerto, and they also examined the Mojokerto skull. They found that volcanic material was still stuck to its base, and Swisher borrowed Curtis' pocket knife to pry some away. The small sample from the skull proved to be too poor in radioactive potassium to provide a reliable age, but in chemical and mineralogical composition it closely matched larger samples collected in the field. When Swisher analyzed the larger samples, he got an age of 1.81 million years, only slightly younger than Curtis's original result.
While Swisher was in Java, he also collected volcanic samples from near Sangiran, which has now provided more than thirty erectus fossils, and he obtained an age of 1.65 million years. If the Mojokerto and Sangiran dates are taken at face value, they imply that Homo erec-tus reached Java about the same time that Homo ergaster emerged in eastern Africa. In this event, we would either have to abandon the species distinction between ergaster and erectus or we would have to argue that they shared an even older and as yet unidentified common ancestor. This ancestor could even have lived in eastern Asia rather than eastern Africa.
So why not accept the dates and revise our understanding of human evolution? Mainly because we lack fundamental stratigraphic observations at either Mojokerto or Sangiran. The Mojokerto date is clearly the more persuasive one, because it is based on volcanic material like that still attached to the skull, but experience in eastern Africa shows that older volcanic particles can be introduced by stream action into much younger deposits, and thorough fieldwork is necessary to detect the possibility of such redeposition. To assess the relevance of both the Mojokerto and Sangiran dates, we would need to know, for example, whether volcanic samples from stratigraphically superimposed horizons provide stratigraphically consistent dates, that is, whether deeper layers provide consistently older dates. If not, redeposition is strongly suggested, and the date on any given layer may overestimate the time of its formation, perhaps by a substantial interval.
The 1.81- to 1.65-million-year ages for Mojokerto and Sangiran erectus cannot be simply dismissed, but they contradict other age estimates for the same deposits based on animal fossils, paleomagnetism, and fission-track dating. The fission-track method is a cousin of potassium/argon dating that depends on the radioactive decay of naturally occurring uranium within ancient volcanic rocks or tektites, and like the potassium/argon method it estimates the last time the rocks were heated to a very high temperature. If the Javan fission-track dates are correct, the Mojokerto and Sangiran erectus fossils are unlikely to be older than 1 million years. Fossils of erectus are also known from China, and so far, the oldest reliable Chinese sites are dated to only about 1 million years or slightly before. For the moment then, there is no persuasive reason to doubt the postulated descent of erectus from ergaster.
East Asian Homo erectus shows that a human species had left Africa by 1 million years ago, and we believe that this species was Homo ergaster. But aside from the issue of the kind of people involved, we may also ask why they left and what route(s) they took. Unlike many other questions in paleoanthropology, these are relatively easy to answer. Archeology shows that about 1.5 million years ago, shortly after ergaster emerged in Africa, people more intensively occupied the drier peripheries of lake basins on the floor of the Great Rift Valley, and they colonized the Ethiopian high plateau (at 2300 to 2400 meters or 7600 to 7700 feet above sea level) for the first time. By 1 million years ago, they had extended their range to the far northern and southern margins of Africa. The Sahara Desert might seem to provide an impenetrable barrier to movement northward, but during the long Acheulean time interval, there were numerous periods when it was somewhat moister and more hospitable, and Acheulean people penetrated it readily.
As to how and why people expanded through Africa and beyond, they almost certainly did so automatically, simply because their physiology and technology allowed them to inhabit territories that no one had occupied before. A group on the periphery of the human range would periodically outgrow its resource base, and a splinter party would break off and set up shop in empty territory next door. Such a party probably rarely moved far, but given time, the splintering process would inevitably have brought people to the northeastern corner of Africa. From there, members of a breakaway group would have colonized the southwestern corner of Asia without even knowing they had left Africa. From southwestern Asia, the same process of population budding would inevitably lead other groups eastwards towards China and Indonesia or northwards and westwards towards Europe.
In theory, early African emigrants could also have dispersed across the Strait of Gibraltar, the Bab-el-Mandeb Strait at the southern end of the Red Sea, or even by island hopping across the central part of the Mediterranean Sea. Each of these routes would require seaworthy boats, however, even during those repeated intervals when the great continental ice sheets sucked water from the world ocean and sea level dropped by 140 meters (460 feet) or more. There is no unequivocal evidence for such boats until after 60,000 years ago, when modern humans must have used them to cross the sea from southeastern Asia to Australia.
The first people to leave Africa crossed the border between what is now Egypt and Israel. It is not surprising therefore that Israel contains the oldest firmly documented archeological site outside of Africa. This occurs at 'Ubeidiya in the Jordan Rift Valley, where ancient lake and river deposits have provided nearly eight thousand flaked stones. The tools include hand axes and other pieces that closely resemble early Acheulean artifacts from Olduvai Gorge and other African sites. They have been bracketed in the interval between 1.4 and 1 million years ago by associated mammal fossils, paleomagnetism, and potassium/argon dating of an overlying lava flow.
Most of the mammal species at 'Ubeidiya are Eurasian, but some are African, and this reminds us of just how close Israel is to Africa. During the long time span of human evolution, Israel was repeatedly invaded by African animal species, mainly during the warmer periods between the longer times of great ice sheet expansion. (During the last such warm period, between about 125,000 and 90,000 years ago, the African immigrants included early modern or near-modern humans.) This raises the possibility that 'Ubeidiya marks a slight, transient ecological enlargement of Africa more than a true human dispersal to Eurasia. If we want to demonstrate a genuine dispersal, we have to look further afield.
Eastern Asia with its Homo erectus fossils shows that such a dispersal must have occurred by 1 million years ago. Europe may have been occupied equally early, but the oldest widely accepted evidence for human colonization is only about 800,000 years old. The evidence comes from the Gran Dolina, a cave at Atapuerca, near Burgos, Spain, that we discuss in the next chapter. Elsewhere in Europe, there is little or no indication that people were present before about 500,000 years ago, and it was perhaps only then that people gained a permanent foothold. Europeans at 500,000 to 400,000 years ago looked a lot like their African contemporaries, and they made similar Acheulean artifacts. They may thus signal a fresh wave of African immigrants.
Considering only east Asian and European fossils and artifacts, we might conclude that people expanded from Africa (beyond Israel) only about a million years ago or a little before. A spectacular discovery at the site of Dmanisi, Republic of Georgia, has recently shown that that conclusion may be premature. Dmanisi is a ruined medieval fortress that Georgian historical archeologists have excavated for many years. In 1984, they broke through the foundation of a medieval structure into an ancient river deposit with animal bones and flaked stone artifacts. Follow-up excavations have produced more than one thousand artifacts and two thousand bones, and the bones include two partial human skulls (Figure 4.9), two lower jaws, and a bone from the sole of the foot. The skulls closely resemble those of Homo ergaster from eastern Africa, but Dmanisi is 1500 kilometers (900 miles) north of 'Ubeidiya, between the Greater and Lesser Caucasus Mountain chains (Figure 4.7). There is thus no question that it marks an early Out-of-Africa dispersal, although there is a question about how early.
Potassium/argon analysis shows that a volcanic basalt at the base of the Dmanisi deposits formed about 1.85 million years ago, If this date is correct, the basalt formed during the Olduvai Normal Paleomagnetic Subchron between 1.95 and 1.77 million years ago (Figure 3.3 on p. 68), and the basalt itself should exhibit normal
Skull No. 2282 from Dmanisi, Georgia (drawn by Kathryn Cruz-Uribe from photographs). (Copyright Kathryn Cruz-Uribe.)
Skull No. 2282 from Dmanisi, Georgia (drawn by Kathryn Cruz-Uribe from photographs). (Copyright Kathryn Cruz-Uribe.)
polarity. It does, and so do the overlying river deposits, which contain the fossils and artifacts. Since the surface of the basalt is fresh, the river deposits probably covered it shortly after it cooled, and they probably also date to the Olduvai Subchron, before 1.77 million years ago. With this in mind, the Dmanisi ergaster fossils could be as old as any in Africa. There is a catch, however. The human fossils and those of other animals occur in large hollows eroded within the normally magnetized river deposits, and the hollows are filled with deposits that exhibit reversed magnetism. The fossils must then be younger than 1.77 million years, and based on paleomagnetism alone, they could date from anytime between 1.77 million and 780,000 years ago, the last time when Earth's magnetic field was reversed. The Dmanisi mammals are said to imply an age closer to 1.77 million years ago, but they represent a unique mix of species, some of which would be the youngest known records of their occurrence, while others would be the oldest. Continued fieldwork may show that two separate species assemblages have been inadvertently mixed, and if so, additional work will be necessary to show which assemblage includes Homo ergaster.
The Dmanisi artifacts include only flakes and flaked pebbles. There are no hand axes, and this could mean that the site formed before Africans invented hand axes roughly 1.7 to 1.6 million years ago. However, even long after this time, not all sites in Africa and Europe contain hand axes, and the reason is obscure. The 800,000-year-old layers at the Gran Dolina, Spain, are an example, and others occur after 500,000 years ago in the same parts of southern and western Europe that hand axe makers had widely settled. In short, the absence of hand axes at Dmanisi need not mean that the people were pre-Acheuleans, and the Dmanisi artifacts require more detailed description to determine whether they differed from Acheulean artifacts in other respects. There is the further problem that different publications on Dmanisi present inconsistent descriptions of the stratigraphic relationship between the artifacts and fossils.
The antiquity of human presence at Dmanisi thus remains an open question. If future research demonstrates that the human bones and artifacts date to 1.77 million years ago, Homo ergaster must have left
Africa almost as soon as it appeared, and we will be forced to speculate on how people could expand so far northwards and not manage to reach Europe for perhaps another million years. If the age of Dmanisi is closer to 1 million years, the gap before the initial occupation of Europe would be much smaller, and the Dmanisi skulls would imply that ergaster remained essentially unchanged for hundreds of thousands of years.
Excepting the Dmanisi skulls, there are only two others between 1.5 million and 600,000 years old that bear on the question of evolutionary change within ergaster. These are a partial skull from Olduvai Gorge which is thought to be roughly 1.2 million years old, and a nearly complete skull from Buia, near the Red Sea coast in Eritrea, eastern Africa, which is about 1 million years old. The Olduvai skullcap is like those of erectus in its massive browridge and thick walls, but in other, more detailed characteristics it is ergaster-like. The Buia skull differs from earlier ergaster skulls only in its somewhat thicker browridge, and it presents a clearer case for long-term anatomical continuity.
By 600,000 to 500,000 years ago, people with larger, more modern-looking braincases had appeared in Africa, and for the moment, based in part on our reading of the artifactual record, we hypothesize that these people evolved abruptly from ergaster. They closely resembled Europeans of 500,000 to 400,000 years ago, and the Africans and Europeans together have sometimes been assigned to the species Homo heidelbergensis, named for a lower jaw found in 1907 in a sand quarry at Mauer near Heidelberg, Germany. It may have been heidelbergensis expanding from Africa about 500,000 years ago that brought the Acheulean tradition to Europe.
In the next chapter, we suggest that Homo heidelbergensis represents the last shared ancestor of the Neanderthals, who evolved in Europe after 500,000 years ago, and of modern humans, who evolved in Africa over the same interval (Figure 4.3). And in future chapters we stress fossil and archeological evidence that modern humans expanded from Africa after 50,000 years ago to swamp or replace the Neanderthals in Europe. But what then of Homo erectus, who was firmly established in eastern Asia long before the Neanderthal and modern human lines diverged? The issue is difficult to address, because relevant east Asian fossils and artifacts are sparser than European ones, and they are more poorly dated. Still, the available fossil and archeo-logical evidence indicates that erectus continued on its own divergent evolutionary trajectory after 500,000 years ago, when Neanderthals and modern humans had separated in the west. This suggests that it eventually suffered the same fate as the Neanderthals.
The most telling late erectus fossils come from the site of Ngandong on the Solo River near Trinil in central Java. Here, between 1931 and 1933, excavations in ancient river deposits by the Dutch Geological Survey in Java recovered more than 25,000 fossil bones, including twelve partially complete human skulls and two incomplete human shin bones. Between 1976 and 1980, researchers from Gadjah Mada University in Yogyakarta expanded the excavations at Ngandong and unearthed 1200 additional bones, including two incomplete human skulls and some human pelvis fragments. Previously, in 1973, the same research team had recovered a similar skull and a human shin bone from like-aged river deposits near Sambungmacan, between Trinil and Sangiran. The Ngandong and Sambungmacan skulls are somewhat larger than those of classic Indonesian erectus, but they exhibit the same basic characteristics, including a massive, shelf-like browridge, a flat, receding forehead, thick skull bone, a tendency for the skull walls to slope inwards from a broad base, and substantial angularity at the rear (Figure 4.10). Based on these features, the Ngandong and Sambungmacan people are commonly assigned to an evolved variant of erectus.
Associated mammal species indicate that the Ngandong and Sambungmacan human fossils are less than 300,000 years old, and they may be much younger. In 1996, the same Berkeley Geochronology Laboratory that provided the 1.81- to 1.65-million-year ages for erectus at Mojokerto and Sangiran, announced that fossil water buffalo teeth associated with the Ngandong and Sambungmacan skulls were between 53,000 and 27,000 years old. This estimate was based on the Electron Spin Resonance method, commonly abbreviated as ESR. ESR depends on the observation that flaws in the crystalline structure of dental enamel accumulate electrons in direct proportion to radioactivity in the burial environment. The principal sources of radioactivity are tiny but nearly ubiquitous amounts of naturally occurring uranium, thorium, and radioactive potassium. ESR is essentially a laboratory technique for measuring the number of trapped electrons. The yearly rate of irradiation, or "annual radiation dose," can be measured in the field, and if we assume that it has remained constant through time, the number of trapped electrons directly reflects the number of years since burial.
In practice, ESR faces many hurdles, of which the most serious is the possibility that teeth at any given site have experienced a flat, receding forehead shelf-like browridge skull broad across the ^Ju^ur™ base oaft srkeualrl
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